In general, a unit cell (synonymous to the battery cell described later) such as a lithium ion secondary battery is used in a battery system in the form of the combination of a plurality of unit cells. The battery system includes, for example, a plurality of groups (hereinafter, this group will be referred to as a “battery module”) that includes a cell group having a plurality of unit cells and a plurality of charging detector CMUs monitoring the cell groups and an assembled battery controller BCU (synonymous to the BMU described later) that controls the plurality of battery modules (for example, see PTL 1). The BCU is connected to the plurality of CMUs respectively installed in the battery modules through an interconnection which is used to transmit and receive data therethrough. Then, each CMU measures a value such as a voltage or a current of the corresponding battery cell, and transmits information on the unit cell corresponding to the measurement result to the BCU through the interconnection. Further, the BCU calculates the state of charge (SOC) and the like of each unit cell based on the information of each unit cell received from each CMU, and determines whether the abnormality occurs by detecting the status of each unit cell.